In commonly owned U.S. Pat. No. 4,081,611 there has been disclosed a PCM station of this type, acting under the control of a central processor as a coupling network or transit exchange between incoming and outgoing links, in which arriving messages are fed to a line unit via a receiving interface while departing messages pass through a transmitting interface. The line unit comprises a resynchronization circuit including a pair of flexible registers forming part of an expandable memory as described in an earlier commonly owned U.S. Pat. No. 3,928,725; another expandable memory has been disclosed in commonly owned U.S. Pat. No. 4,058,682.
In such a PCM/TDM system, international regulations call for the organization of transmitted data in a succession of frames each consisting of a multiplicity of channels accommodating multibit words, usually 8-bit bytes; a channel may be defined as a time slot subdivided into as many clock cycles or phases as there are bits in a word. For proper synchronization of the clocks controlling the operation at the transmitting and receiving ends of a signal path, certain alignment words are used in an initial channel of each frame which may be referred to as the No. 0 channel. These alignment words generally differ from each other in odd-numbered or "first" and even-numbered or "second" frames F.sub.A, F.sub.B ; thus, they will alternately assume two different forms referred to hereinafter as "word A" and "word B". Word "A", appearing in the No. 0 channel of the recurrent first frame F.sub.A, may have a large number of its bits (e.g. 7 out of 8) arranged in an invariable configuration facilitating the recognition of that word by a detector at the receiving end; word "B", present in the No. 0 channel of the immediately following second frame F.sub.B, need only have one particular bit in a predetermined time position to serve as a confirmation of alignment.
Reference may be made to my concurrently filed application Ser. No. 278,064 for a description of a frame former generating these alignment words in a transmitting section of a PCM station.